Anionic photoinitiation

ABSTRACT

A method for the anionic photoinitiation of polymerization or crosslinking of a substrate by irradiating a mixture of a substrate and a photoinitiator, where the photoinitiator is an inorganic transition metal complex which releases on irradiation a polymerization initiating substance consisting essentially of an anionically charged nucleophile that initiates the polymerization or crosslinking reaction, under conditions that allow the anion to initiate polymerization.

The government has rights in this invention by virtue of the partialfunding of work leading to this invention through National ScienceFoundation Grant No. DMR-8715635.

This application is a continuation in part of U.S. patent applicationSer. No. 07/790,355, filed on Nov. 12, 1991, now abandoned, by CharlesR. Kutal, entitled, "Anionic Photoinitiation".

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term substrate refers to the compound, monomer,oligomer, or polymer that is acted on by the anionic photoinitiator. Ingeneral, the photoinitiator causes a change in the chemical compositionand/or physical structure of the substrate, to produce a product (seeequations 2-3).

As used herein, the term inorganic complex refers to an inorganiccompound that contains a ligand that is capable of being released as anucleophilic anion on irradiation with light of the appropriatewavelength.

As used herein, the term nucleophile refers to an electron rich moietythat will attack the nucleus of a carbon atom.

The term alkyl, as used herein, unless otherwise specified, refers to asaturated straight or branched hydrocarbon of C₁ to C₁₀, andspecifically includes methyl, ethyl, propyl, isopropyl, butyl, isobutyl,t-butyl, pentyl, cyclopentyl, isopentyl, neopentyl, hexyl, isohexyl, andcyclohexyl.

The terms alkenyl and alkynyl, as used herein, refers to a C₂ to C₁₀straight or branched hydrocarbon with at least one double or triplebond, respectively. phenyl, which can be substituted with one or morealiphatic groups.

The term aliphatic refers to a hydrocarbon, typically of C₁ to C₁₀, thatcan contain one or a combination of alkyl, alkenyl, or alkynyl moieties,and which can be straight, branched, or cyclic, or a combinationthereof.

A method for anionic photoinitiation is provided that includes exposingthe substrate to be polymerized or crosslinked to an inorganictransition metal complex that releases anions in a controlled fashionwhen irradiated. The metal complex is thermally stable, soluble in thereactants, and nonreactive in the absence of light.

The anionic photoinitiators can be used to prepare new classes ofphotoresponsive materials possessing optimal properties for specificapplications, such as improved thermal stability, photosensitivity, andwavelength response. Such materials are needed in the microelectronics,coatings, and reprographic industries, specifically for lithographicphotoresists, photocurable coatings, stereolithographic media, sealants,printing inks and adhesives.

The anionic photoinitiation of selected substrates can be characterizedby absorption and luminescence spectroscopies, real-time Fouriertransform infrared spectroscopy, quantum yield measurements, flashphotolysis, reactive-ion-etching, and scanning electron microscopy.

Anionic polymerization itself has been known, extensively described, andwell categorized in the literature for years. Specifically, theconditions under which anionic polymerization occurs, and the conditionsunder which it is quenched, have been studied and documented. See forexample, Principles of Polymerization, by George Odian, Second Edition,1981, page 372-402; and The Encyclopedia of Polymer Science andEngineering, Volume 2, "Anionic Polymerization to CationicPolymerization," John Wiley and Sons, N.Y., 1985, pages 1-42. Theselection of appropriate reaction systems (solvents and inhibitors ortraps) which promote, inhibit, or prevent anionic radical and othertypes of polymerizations are also well known.

In one embodiment of the invention, a method for the anionicphotoinitiation of polymerization or crosslinking of a substrate ispresented that includes the step of irradiating a mixture of a substrateand an inorganic transition metal complex, wherein the inorganic complexreleases on irradiation a polymerization initiating substance consistingessentially of an anionically charged nucleophile that initiates thepolymerization or crosslinking reaction, under conditions that allow theanion to initiate polymerization. This embodiment does not preclude thepresence of radicals or other species in the reaction mixture, but onlythat the radicals or other species do not significantly initiatepolymerization. One of ordinary skill in the art can easily determinewhich species is being released at a given wavelength, and which speciesis the polymerization initiating substance. For example, the release ofanionic chlorine can be measured with a chlorine ion specific electrodeor colorometrically by titration. The release of ethylenediamine can bemeasured for example by assessing the change in pH over time or by useof reagents that react with amines to produce fluorescent products thatcan be quantitated. If it is determined that either of these species,for example, is released under the conditions of polymerization, thenaddition of more of that species ought to have an effect onpolymerization.

I. Selection of Inorganic Complex as Anionic Photoinitiator

A. Method of Selection of the Inorganic Complex as an AnionicPhotoinitiator

An inorganic transition metal complex should be chosen for use as ananionic photoinitiator that has the following characteristics.

1) The inorganic transition metal complex must release a nucleophilicanion on irradiation. The ability of the complex to initiate anionicpolymerization or crosslinking is a function of the nucleophilicity ofthe released anion and the reactivity of the monomer or other substrate.In general, the nucleophilicity of anions in aqueous or alcoholicsolution decreases in the order: CN⁻ >I-->NCS⁻ >OH⁻ >Br⁻ >N₃ ⁻ >NO₂ ⁻>Cl⁻ >F⁻. The complex may also release ⁻ OR, wherein R is a straight orbranched alkyl group, including but not limited to methyl, ethyl,propyl, butyl, and pentyl (linear and branched analogs). This order willchange in nonhydroxylic solvents owing to differences in solvationenergies.

Metal complexes that photochemically liberate hydroxide ion areparticularly attractive, in light of the fact that this strong base iscapable of initiating anionic polymerization or crosslinking ofepoxides, as well as other moderately reactive species such assiloxanes. For example, addition of OH⁻ to the oxirane ring produces areactive anionic species that can then attack a second ring. Repetitionof this process results in polymerization or crosslinking with higheffective quantum efficiency.

The presence of the anionic species and the use of an anionic mechanismcan be easily determined. Successful polymerization, in the presence ofa known radical scavenger, such as hydroquinone or oxygen, clearlyindicates that a radical mechanism is not involved. Further confirmationof the anionic mechanism is provided by attempts to polymerize thesubstrate in the presence of an anion scavenger, such as an acid.

2) The release of the anion should be the result of excitation of theinorganic transition metal complex by visible, near-infrared, orultra-violet light. Light of wavelength between approximately 400 and750 nm is visible. Light of wavelength between 750 and 1000 nm isconsidered near-infrared radiation. Light of wavelength between 200 and400 nm is considered ultraviolet radiation.

Generally, the absorption process causes the transfer of an electronfrom a molecular orbital with bonding or nonbonding character to amolecular orbital with antibonding character with respect to the centralmetal-anion bond. In the typical case, the inorganic metal complexundergoes a ligand field or a ligand-to-metal charge transfer transitionwhen irradiated with light of a specific wavelength. The wavelength atwhich the absorption takes place can be determined by analysis of thecomplex's ultraviolet, visible, and near-infrared spectrum. For example,ligand-field absorption maxima occur at 392 and 520 nm fortrans-Cr(NH₃)₂ (NCS)₄ ⁻, at 365, 414 and 487 nm for trans-Cr(en)₂ (NCS)₂⁺, and at 406 and 619 nm for trans-Co(en)₂ Cl₂ ⁺ in aqueous solution.

The ligand field absorbance typically occurs at higher wavelength (lowerenergy) than the ligand to metal charge transfer band for the lightertransition metals (first and second rows). When transition metalcomplexes absorb energy in the ligand field region, they release ligandsfrom the complex, but do not normally release free radical species. Whentransition metal complexes absorb energy in the ligand to metal chargetransfer region, the complex releases a number of species, includingpossibly neutral species, anions, and free radicals. Given this basicknowledge that is elementary to one skilled in the art, appropriatereaction conditions and monomers can be selected to prevent radicalsfrom and allow anions to initiate polymerization.

The ability to initiate useful chemistry when excited withlong-wavelength light is a highly desirable feature of transition metalcomplexes. This can be used, for example, in photosensitive systems thatrespond to the output of inexpensive semiconductor-diode lasers.

3) The release of the anion from the transition metal complex shouldoccur with high quantum efficiency, preferably 0.01 or larger. The term"quantum efficiency" refers to the ratio of the number of anionsgenerated per photon absorbed by the complex. The numerator in thisratio, the moles of released anion, can be determined by severalanalytical techniques such as colorimetry, gravimetry, and specific-ionelectrode measurements. The denominator of the quantum yield ratio, themoles of absorbed photons, is obtained by measuring the incident lightintensity with a calibrated power meter or chemical actinometer such asferrioxalate and then calculating the percentage of this radiationabsorbed by the complex by use of Beer's Law.

Quantum efficiencies for photochemical release of anions from transitionmetal complexes vary depending upon the complex and reaction conditions.Irradiation of an acetonitrile solution of trans-Cr(NH₃)₂ (NCS)⁻ ₄ with465-nm light causes NCS⁻ release with a quantum efficiency of 0.26.Photolysis of an aqueous solution of trans-Co(en)₂ Cl₂ ⁺ with 488-nmlight generates Cl⁻ with a quantum efficiency of 0.0011. While quantumefficiencies of anion release near unity are desirable, values as low as0.0001 may be acceptable in systems that incorporate chemicalamplification. The effective quantum efficiency in achemically-amplified system will be the product of the quantumefficiency of the photochemical step (eq. 2) and the average chainlength or number of substrate units reacted per photogeneratedinitiator/catalyst species (eq. 3).

The number of substrate units undergoing polymerization or crosslinkingcan be obtained from gel permeation chromatography measurements ofnumber and weight average molecular weights. Consumption ofcarbon-carbon double bonds in vinylic substrates can be quantitated byreal time Fourier transform spectrometry.

EXAMPLE 1

Multiplicity of Response to Reineckate's Anion by Ethyl α-Cyanoacrylate

FIG. 3 is a plot of percentage polymerization vs. time of irradiationfor samples of ethyl α-cyanoacrylate containing the followingconcentrations of Reineckate's anion: (□) O M; (*) 2.2×10⁻⁴ M; (+)5.4×10⁻⁴ M; (▪) 2.0×10⁻³ M. Samples were coated onto acid-treatedsilicon wafers and irradiated with 500 mW/cm² of polychromatic (300-630nm) light. Under the rather conservative assumption that eachReineckate's anion in the sample undergoes photochemical release of oneNCS⁻, a polymerization chain length of >10⁴ was calculated from the datain FIG. 1 for the 2.2×10⁻⁴ M sample after irradiation for 30 seconds.This multiplicative response of ethyl α-cyanoacrylate to NCS⁻constitutes chemical amplification and accounts for the highphotosensitivity of this system.

4) The inorganic metal complex should be soluble in the pure substrateor in a solvent that also dissolves the substrate. Solubility of thecomplex in nonaqueous solutions is necessary to enable formulation witha range of organic substrates such as epoxides and acrylates. A typicalphotoresist formulation used for photolithography consists of an organicsubstrate, a photoinitiator, and a casting solvent. This formulation isspin-coated onto a silicon wafer to form a thin film and the solvent isthen removed by heating. Common casting solvents include 2-butanone,cyclohexanone, and propylene glycol methyl ether acetate. Forpolymerization reactions, solubility of the inorganic photoinitiator inthe monomer obviates the need for a solvent.

5) The inorganic metal complex should be thermally stable for areasonable length of time, to provide a suitable shelf life. For manyapplications, a shelf life of 3-6 months at room temperature isdesirable. Transition metal complexes in which the central metalpossesses a d³, low-spin d⁶, or low-spin d⁸ electronic configuration areespecially resistant to thermal release of ligands. Other examples ofthermally stable complexes include those with metals in high formaloxidation states such as Mo(CN)₈ ⁻⁴. Shelf life is improved byminimizing the exposure of the complex to heat or light.Photopolymerizable solutions of trans-Cr(NH₃)₂ (NCS)₄ ⁻ in ethylα-cyanoacrylate remain usable for up to one month when stored in thedark at room temperature.

One approach for retarding the thermal substitution reactions ofcoordination complexes involves tethering the central metal with amacrocyclic ligand. For example, trans-Cr(cyclam)Cl₂ ⁺ (cyclam is1,4,8,11-tetraazacyclotetradecane) undergoes thermal release of Cl⁻ ˜10³times more slowly than trans-Cr(en)₂ Cl₂ ⁺. While the quantum yield forphotoinduced Cl⁻ loss experiences a comparable reduction, this factor iseasily compensated by employing a more intense light source (e.g. alaser rather than a conventional arc lamp).

B. Examples of Anionic Photoinitiators Reineckate's Anion

Reineckate's anion, trans-[Cr(NH₃)₂ (NCS)₄ ]⁻, is a preferred transitionmetal complex for anionic photoinitiation. The anion dissolves readilyin a variety of nonaqueous solvents, resists thermal substitution innonhydroxylic media, and undergoes quantum efficient release of thenucleophile NCS⁻ upon ligand-field excitation withnear-ultraviolet/visible light at greater than 350 nm. Photochemicalrelease of NH₃ or radicals from the Reineckate anion only occurs at muchshorter wavelengths.

The photogenerated thiocyanate anion will attack an electron deficientcarbon to create a new anionic reactive center.

    Trans-Cr(en).sub.2 (NCS).sub.2.sup.+

Trans-Cr(en)₂ (NCS)₂ ⁺, a cationic analogue of Reineckate's salt,undergoes photochemical release of NCS⁻ under green light, and thereforecan function as a photoinitiator for anionic polymerization. The complexis soluble in acetonitrile and acetone.

EXAMPLE 2

Anionic Polymerization of Ethyl α-Cyanoacrylate with Trans-Cr(en)₂(NCS)₂ ⁺

A solution containing 95% by volume of ethyl α-cyanoacrylate, 5% byvolume of acetonitrile, and 1.1×10⁻³ M trans-[Cr(en)₂ (NCS)₂ ]PF₆ wasexposed to 532-nm light from a Nd-YAG laser at an intensity of 2.9×10⁻⁶einsteins/s. A solid plug of polymer formed in the path of the lightbeam after irradiation for 90 s.

    Trans-Cr(en).sub.2 Cl.sub.2.sup.+

Trans-[Cr(en)₂ Cl₂ ]Cl can be prepared by the method described by G.Brauer in Handbook of Preparative Inorganic Chemistry, 2nd Ed., vol. 2,Academic Press: New York, 1965, p. 1357-1358. Simple metathesis with NH₄PF₆ converts the complex to its PF₆ ⁻ salt. Irradiation into the firstspin-allowed ligand field absorption band of trans-Cr(en)₂ Cl₂ ⁺ isknown to cause release of Cl⁻ with a quantum efficiency of 0.32 inaqueous solution (see Canadian Journal of Chemistry, vol. 48, 1524,1971). It has been discovered that Cl⁻ is an effective initiator for theanionic polymerization of ethyl α-cyanoacrylate. Thus as little as1.3×10⁻⁴ M Cl⁻ causes 0.5 mL of ethyl α-cyanoacrylate to polymerize inless than 1 second. A 532-nm irradiation of a solution containing 95% byvolume of ethyl α-cyanoacrylate, 5% by volume of acetonitrile asco-solvent, and 2.0×10² M trans-[Cr(en)₂ Cl₂ ]PF₆ can be used to inducephotoinitiated anionic polymerization of the acrylate monomer.

    Pt(acac).sub.2

It has been discovered that Pt(acac)₂ at concentrations as low as 105ppm and wavelengths slightly less than 300 nm to greater than 300 nm areeffective in initiating anionic polymerization reactions. The photolysistime required for polymerization depends upon several variables,including Pt(acac)₂ concentration, light intensity and excitationwavelength.

The electronic structure and spectroscopy of Pt(acac)2 has beendescribed in Inorg. Chem., 1993, 32, 1251-1258. It appears thatirradiation of Pt(acac)₂ initially excites a ligand-localized π--π*transition rather than a ligand field or ligand-to metal charge transfertransition. The complex releases an anionic polymerization initiatingspecies on irradiation, that is probably acac-.

For example, photolysis (λ>300 nm) of a 1.07×10⁻⁴ M (42 ppm) Pt(acac)₂methanolic solution for six minutes resulted in the loss of discerniblebands due to Pt(acac)₂ and the formation of a single intense absorptioncentered at 272 nm. The ultraviolet\visible spectra of the material alsoindicated the formation of colloidal platinum. The shape and position ofthe absorption band at 272 nm, that was formed as a result ofphotolysis, are consistent with the UV/Vis spectrum of an authenticsample of 2,4-pentanedione in methanol. This indicates that the speciesresponsible for initiating polymerization is the photo-liberatedacetylacetonate anion.

EXAMPLE 3

Anionic Polymerization of Ethyl α-Cyanoacrylate with Pt(acac)₂

A solution containing 95% by volume of 2-cyanoethylacrylate, 5% byvolume of acetonitrile, and varying concentrations Pt(acac)₂ ("PA") wasexposed to light from a 200-watt high pressure mercury lamp. The timefor polymerization of the 2-cyanoethylacrylate for the variousconcentrations of photoinitiator is shown in Table 1 below.

                  TABLE 1    ______________________________________    Amount of PA   Excitation       time for    (M/ppm)        wavelength.sup.a polymerization    ______________________________________     3.81 × 10.sup.-3 /1500                   >300    nm       21.9  sec.    1.27 × 10.sup.-3 /500                   >300    nm       25.7  sec.    1.14 × 10.sup.-3 /448                   >300    nm       17.3  sec.    8.52 × 10.sup.-4 /335                   >300    nm       20.6  sec.    7.25 × 10.sup.-4 /285                   >300    nm       27.2  sec.    2.67 × 10.sup.-4 /105                   >300    nm       67.2  sec.    2.67 × 10.sup.-4 /105                   365     nm       211.1 sec.    ______________________________________     .sup.a The output from a 200watt highpressure mercury lamp was passed     through Pyrex to obtain λ >300 nm or through a narrow bandpass     interference filter to isolate 365 nm light.

The anionic mechanism of these polymerizations was confirmed by carryingout the reaction with a known free radical scavenger, hydroquinone, anda known anion scavenger, methyl sulfonic acid. The photolysis timerequired to initiate 2-cyanoethylacrylate polymerization was unchangedin the presence of a large excess of hydroquinone. In contrast,polymerization was inhibited in samples when the concentration of ananion scavenger, methyl sulfonic acid, was increased to 100 ppm orgreater.

                  TABLE 2    ______________________________________                          Inhibitor.sup.b                                       time for    Amount of PA               Excitation.sup.a                          Concentration                                       polymer-    (M/ppm)    wavelength (M/ppm)      ization    ______________________________________    1.79 × 10.sup.-3 /704               >300 nm    0            14.5 sec.    1.79 × 10.sup.-3 /704               >300 nm    4.28 × 10.sup.-4 /41.sup.c                                       55.8 sec.    8.52 × 10.sup.-4 /335               >300 nm    0            20.6 sec.    8.52 × 10.sup.-4 /335               >300 nm    4.28 × 10.sup.-3 /411.sup.c                                       >600 sec.    1.14 × 10.sup.-3 /448               >300 nm    0            17.3 sec.    1.14 × 10.sup.-3 /448               >300 nm    4.28 × 10.sup.-3 /411.sup.c                                       >600 sec.    7.25 × 10.sup.-4 /285               >300 nm    0            27.2 sec.    7.25 × 10.sup.-4 /285               >300 nm     1.56 × 10.sup.-2 /1498.sup.c                                       >600 sec.     3.81 × 10.sup.-3 /1500               >300 nm    0            21.9 sec.     3.81 × 10.sup.-3 /1500               >300 nm     3.18 × 10.sup.-2 /3500.sup.d                                       23.2 sec.    ______________________________________     .sup.a The output from a 200watt highpressure mercury lamp was passed     through Pyrex to obtain λ >300 nm or through a narrow bandpass     interference filter to isolate 365 nm light.     .sup.b Concentrations do not take into account the inhibitor     concentrations that are already present in 2cyanoethylacrylate.     2cyanoethylacrylate, as received, contains 5 ppm of the anion inhibitor     methyl sulfonic acid and 1000 ppm of the freeradical scavenger     hydroquinone.     .sup.c Concentration of methyl sulfonic acid.     .sup.d Concentration of hydroquinone.

As with the other anionic photoinitiators described herein, samples of2-cyanoethylacrylate containing Pt(acac)₂ were stable in the dark. Forthis system, no apparent decomposition or polymerization occurred after6 days. When the methylsulfonic acid concentration in2-cyanoethylacrylate samples containing Pt(acac)2 was raised from 5 ppmto 40 ppm, samples were stable in the dark for at least 40 days withoutsignificant decrease in photosensitivity.

Cyclopentadienyl Metallo Complexes

It has also been discovered that iron and ruthenium cyclopentadienylcomplexes can act as polymerization initiating species for anionicpolymerization. The cyclopentadientyl groups can be unsubstituted, orone or both of the cyclopentadienyl groups can be independentlysubstituted with one, two, or more substituents of the formula --C(O)R,wherein R is a C₁ to C₁₀ aliphatic group (alkyl, alkenyl, or alkynyl) oraryl. Typical wavelengths for photoinitiation using these complexesrange from 290 to 500 nm. The optimal wavelength of irradiation for eachcomplex can be determined by simple inspection of theultraviolet/visible spectrum for the complex.

The photochemistry of complexes such as (η⁵ --C₅ H₅) (η⁵ --C₅ H₄ COR)₂Fe (where R=C₆ H₅, H, CH₃) and (η⁵ --C₅ H₄ COR)₂ Fe (where R=C₆ H₅) hasbeen relatively well studied. Photolysis of these complexes, in solventswhich contain trace impurities of water, results in the formation of(π--C₅ H₅ Fe(solv)_(n) ⁺ PhCO₂ ⁻ +C₅ H₄ ⁻ +(η⁵ --C₅ H₅)₂ Fe (as a minorproduct). Either anionic species is capable of initiating anionicpolymerization. FIG. 1 is a graph of the UV-visible spectrum of (η⁵ --C₅H₄ COC₆ H₅)₂ Fe (2.92×10⁻⁴ M or 115 ppm, extinction coefficient365=3,976) in acetonitrile. FIG. 2 is a graph of the UV-visible spectrumof (η⁵ --C₅ H₅)₂ Ru (2.77×10⁻³ M), extinction coefficient 365=264 M⁻¹cm⁻¹).

It is generally accepted that the photolysis of ruthenocene, inrelatively polar solvents, results in oxidation leading to the formationof the ruthenicenium ion and solvated electrons. The species responsiblefor initiating the anionic polymerization of 2-cyanoethylacrylate may bea photoejected solvated electron. Solvated electrons are capable ofinitiating anionic polymerization reactions.

EXAMPLE 4

Anionic Polymerization of Ethyl a-Cyanoacrylate with (η⁵ --C₅ H₅)₂ RU

Solutions containing 95% by volume of ethyl α-cyanoacrylate, 5% byvolume of acetonitrile, and varying concentrations of (η⁵ --C₅ H₅)₂ Ruwas exposed to light having a wavelength of greater than 300 nm from a200-watt high pressure mercury lamp passed through Pyrex. The time forpolymerization of the 2-cyanoethylacrylate for the variousconcentrations is shown in Table 3 below.

                  TABLE 3    ______________________________________    Amount of    Photoinitiator Excitation                             Time for    (M/ppm)        wavelength.sup.a                             polymerization    ______________________________________    1.01 × 10.sup.-2 /2337                   >300 nm   11.3 sec.    5.06 × 10.sup.-3 /1170                   >300 nm   10.9 sec.    2.42 × 10.sup.-3 /560                   >300 nm   16.6 sec.    9.30 × 10.sup.-4 /215                   >300 nm   20.6 sec.    ______________________________________     .sup.a The output from a 200watt highpressure mercury lamp was passed     through Pyrex to obtain λ >300 nm.

The anionic mechanism of the reaction was confirmed by conductingpolymerizations in the presence of a known radical scavenger,hydroquinone, or a known anion scavenger, methylsulfonic acid. Thephotolysis time required to initiate 2-cyanoethylacrylate polymerizationwas relatively unaffected by the presence of a large excess ofhydroquinone. In contrast, polymerization was completely inhibited insamples where the concentration of the anion scavenger, methyl sulfonicacid, had been increased to 296 ppm or greater.

                  TABLE 4    ______________________________________    Anionic and Free-Radical Inhibition Studies               Excit-    Amount of  ation.sup.a                        Inhibitor.sup.b                                       time for    Photoinitiator               wave-    Concentration  polymer-    (M/ppm)    length   (M/ppm)        ization    ______________________________________     1.01 × 10.sup.-2 /2337               >300 nm  0              11.3 sec.     1.01 × 10.sup.-2 /2337               >300 nm   ≈5.14 × 10.sup.-1 /49,367.sup.c                                       >600 sec.    2.42 × 10.sup.-3 /560               >300 nm  0              16.6 sec.    2.42 × 10.sup.-3 /560               >300 nm  5.14 × 10.sup.-3 /494.sup.c                                       >600 sec.    2.42 × 10.sup.-3 /560               >300 nm  3.63 × 10.sup.-3 /400.sup.d                                       18.7 sec.    9.30 × 10.sup.-4 /215               >300 nm  0              20.6 sec.    9.30 × 10.sup.-4 /215               >300 nm  3.08 × 10.sup.-3 /296.sup.c                                       >600 sec    9.30 × 10.sup.-4 /215               >300 nm   1.56 × 10.sup.-2 /2000.sup.d                                       48.0 sec.    9.30 × 10.sup.-4 /215               >300 nm   3.63 × 10.sup.-2 /4000.sup.d                                       75.9 sec.    ______________________________________     .sup.a The output from a 200watt highpressure mercury lamp was passed     through Pyrex to obtain λ >300 nm.     .sup.b Concentrations do not take into account the inhibitor     concentrations that are already present in 2cyanoethylacrylate. Ca, as     received, contains 5 ppm of the anion inhibitor methyl sulfonic acid and     1000 ppm of the freeradical scavenger hydroquinone.     .sup.c Concentration of methyl sulfonic acid.     .sup.d Concentration of hydroquinone.

Samples of 2-cyanoethylacrylate containing (η⁵ --C₅ H₅)₂ Ru were stablein the dark with no apparent decomposition or polymerization occurringfor as long as 14 days.

EXAMPLE 5

Anionic Polymerization of Ethyl α-Cyanoacrylate with (η⁵ --C₅ H₅) (η⁵--C₅ H₄ COR)₂ Fe (where R=C₆ H₅)

Solutions containing 95% by volume of ethyl α-cyanoacrylate, 5% byvolume of acetonitrile, and varying concentrations of (η⁵ --C₅ H₅)(η⁵--C₅ H₄ COR)₂ Fe (where R=C₆ H₅) was exposed to light having awavelength of region λ>320 nm. The time for polymerization of the2-cyanoethylacrylate for the concentrations is shown in Table 5 below.

                  TABLE 5    ______________________________________                                Excit-                                ation    Photoinitiator  Photoinitiator                                Wave-   Time for    (η.sup.5 -C.sub.5 H.sub.4 R')            (η.sup.5 -C.sub.5 H.sub.4 COR)Fe                        concentration                                    length                                          polymer-    R       R           (M/ppm)     (nm)  ization.sup.a    ______________________________________    COC.sub.6 H.sub.5            C.sub.6 H.sub.5                        5.84 × 10.sup.-4 /230                                    >320   4.6 sec.    H       H           1.57 × 10.sup.-3 /335                                    >320  508 sec.    H       CH.sub.3    2.19 × 10.sup.-3 /500                                    >320  421 sec.    H       C.sub.6 H.sub.5                        2.36 × 10.sup.-3 /685                                    >320  196 sec.    ______________________________________     .sup.a It should be noted that 2cyanoethylacrylate, as received, contains     5 ppm of the anion inhibitor methyl sulfonic acid and 1000 ppm of the     freeradical scavenger, hydroquinone.

The anionic mechanism of the polymerization process was again confirmedby carrying out the reaction in the presence of the radical scavengerhydroquinone, and the anion scavenger methyl sulfonic acid. Thephotolysis time required to initiate 2-cyanoethylacrylate polymerizationwas unchanged in the presence of a large excess of hydroquinone. Incontrast, polymerization was inhibited in samples where theconcentration of the anion scavenger, methyl sulfonic acid, had beenincreased to 494 ppm or greater.

As with the other photoiniatiors, samples of 2-cyanoethylacrylatecontaining (η⁵ --C₅ H₅) (η⁵ --C₅ H₄ COR)₂ Fe (where R=C₆ H₅, H, CH₃) or(η⁵ --C₅ H₄ COR)₂ Fe (where R=C₆ H₅) are stable in the dark with noapparent decomposition or polymerization occurring for as long as 14days. In particular, a 2-cyanoethylacrylate sample containing 685 ppm of(η⁵ --C₅ H₅) (η⁵ --C₅ H₄ COR)₂ Fe (where R=C₆ H₅) was stable, in thedark for 20 days.

                                      TABLE 6    __________________________________________________________________________    Photoinitiator  Photoinitiator                             Inhibitor    (η.sup.5 -C.sub.5 H.sub.4 R')           (η.sup.5 -C.sub.5 H.sub.4 COR)Fe                    concentration                             Concentration                                      Time for    R'     R        (M/ppm)  (M/ppm)  polymerization.sup.a    __________________________________________________________________________    COC.sub.6 H.sub.5           C.sub.6 H.sub.5                    5.84 × 10.sup.-4 /230                             5.14 × 10.sup.-3 /494.sup.b    COC.sub.6 H.sub.5           C.sub.6 H.sub.5                    5.84 × 10.sup.-4 /230                             3.94 × 10.sup.-3 /4333.sup.c    H      H        1.57 × 10.sup.-3 /335                             5.14 × 10.sup.-3 /494.sup.b    H      H        1.57 × 10.sup.-3 /335                             2.73 × 10.sup.-3 3000.sup.c    H      CH.sub.3 2.19 × 10.sup.-3 /500                             5.14 × 10.sup.-3 494.sup.b    H      CH.sub.3 2.19 × 10.sup.-3 /500                             2.73 × 10.sup.-3 3000.sup.c    H      C.sub.6 H.sub.5                    2.36 × 10.sup.-3 /685                             5.14 × 10.sup.-3 494.sup.b    H      C.sub.6 H.sub.5                    2.36 × 10.sup.-3 /685                             2.73 × 10.sup.-3 3000.sup.c    __________________________________________________________________________     .sup.a The light source was a 200Watt highpressure mercury lamp, passed     through pyrex to obtain λ >320 nm.     .sup.b Concentration of the anion inhibitor, methyl sulfonic acid.     .sup.c Concentration of the freeradical scavenger, hydroquinone.

II. Examples of Substrates that can be Treated with the AnionicPhotoinitiator

Monomers, oligomers, and polymers that can be modified by anionicphotoinitiation are those that are capable of reaction on attack by ananionically charged nucleophile, including but not limited to ethylene,1,3-dienes, styrene and α-methyl styrene, acrylates and methacrylates,acrylonitrile, methacrylonitrile, acrylamide and methacrylamide, andaldehydes and ketones, and oligomers and polymers containing thesemoieties. Suitable substrates also include monomers, oligomers, andpolymers that undergo anionic ring-opening polymerization orcrosslinking reactions, for example, N-carboxy-α-amino anhydrides,cyclic amides, cyclic esters, epoxides and siloxanes.

Preferred monomers are ethylenically unsaturated monomers that containselectron-withdrawing substituents to stabilize the negative charge, forexample, ethyl α-cyanoacrylate (see equation 5) and methylα-trifluoromethylacrylate. ##STR1##

In general, an anionic photoinitiator is selected that will react withthe substrate to be modified or polymerized. The more nucleophilic theanion released from the excited metal complex, the less electrophilicthe substrate is required to be. For example, ethyl α-cyanoacrylate isquite reactive because the electron-withdrawing effect of the cyanogroup causes the β-carbon to be highly electrophilic. Almost any anionwill initiate polymerization of this monomer, including thiocyanate,bromide, and azide ion. Methyl α-trifluoromethylacrylate is also quitereactive because the α-trifluoromethyl group has a strong inductiveeffect. In contrast, ethylene does not contain an electron withdrawinggroup and thus polymerization of this monomer must be initiated by astrong nucleophile, such as n-butyl carbanion. N-carboxy-α-aminoanhydrides, cyclic amides, cyclic esters, epoxides and siloxanes can beinitiated with alkoxide or hydroxide ion.

Numerous polymer textbooks describe in detail whether monomers areactive toward anionic, cationic, or radical polymerization, or a mixturethereof, and therefore this type information is known, or can be easilydetermined without undue experimentation using standard assays by oneskilled in the art. One of the goals of this invention is to avoid freeradical polymerization, because it is sensitive to atmospheric oxygen.

III. Anionic Photoinitiation Process

The photoinitiator can be dissolved in neat monomer (e.g. Reineckate'ssalt in ethyl α-cyanoacrylate) or in a solution of substrate and aco-solvent (e.g. trans-Cr(en)₂ (NCS)₂ ⁺ in 95% ethyl α-cyanoacrylate and5% acetonitrile).

The anionic process can be carried out by either dissolving thephotoinitiator in neat monomer or in a solution of substrate and aco-solvent. If a co-solvent is employed, it should not undergo protontransfer equilibria, since this will provide a pathway for scavenginganion species active in initiating and propagating polymerization. Waterand aqueous acids are classic examples of solvents that undergo protontransfer equilibria. Water has an especially negative effect on anionicpolymerization, since it is an active chain transfer agent. Examples ofpreferred solvents are diethyl ether, tetrahydrofuran, and acetonitrile.It is typically desired, but not required, that the complex have asolubility of at least 1×10⁴ M in the cosolvent.

The anionic photoinitiation reaction can be carried out at anytemperature that provides the desired results. A temperature of between20°-30° C. is suitable in many cases, since higher temperatures favorthermal release of anions and lower temperatures may cause solubilityproblems.

Any amount of photoinitiator can be used that suitably initiates thedesired reaction. An amount of photoinitiator in the formulation rangingfrom parts per million to part per hundred is typically suitabledepending upon the system and desired applications.

Anionic photoinitiation is retarded in the presence of acids thatneutralize the anion, or nucleophile. Certain unsaturated monomers usedin chain polymerization reactions contain compounds that act as radicalor basic scavengers, that are in fact acids and that can interfere withthe anionic polymerization reaction.

On initiation of an anionic polymerization process, a large number ofmonomer anions are formed that react with other monomers to formpolymers. If a small number of acidic groups are present in thepolymerizing solution, for example, in the form of a stabilizer orcomonomer, those acidic groups will quench an equivalent number ofpropagating monomer anions. The remaining anions will continue to reactwith other monomers to form polymers. Based on the amount of acidicgroups present, the anionic reaction will either be retarded or quenchedcompletely. When a solvent is used that contains an active hydrogen,such as water or acid, since the solvent is almost always present in amolar quantity that exceeds, and usually greatly exceeds, the molarquantity of the anions in solution, the anionic polymerization reactioncannot occur because all anions formed are immediately quenched.

The material with the anionic photoinitiator should be maintained in theabsence of radiation of the wavelength that will initiate reaction,until reaction is desired.

The method is described below for convenience with reference to thereaction of ethyl α-cyanoacrylate with Reineckate's anion. However, itshould be understood that the present invention is not limited to theseexamples but includes all of the embodiments described above.

EXAMPLE 6

Anionic Photoinitiation of α-Cyanoacrylate by Reineckate's Anion

Neat ethyl α-cyanoacrylate (99.88% purity from Loctite Corporation) is acolorless liquid with a viscosity of 2.8 cP at 22° C. The sampleemployed in this study contained scavengers for adventitious radical andbasic impurities. Commercially available ammonium salt of Reineckate'sanion (Alfa) was converted to the potassium salt (K⁺ R⁻). Solutions ofethyl α-cyanoacrylate containing K⁺ R⁻ (0.19-2.7×10⁻³ M) undergo nochange in viscosity when kept in the dark for one week at roomtemperature. In contrast, mixing one drop of a 5.2×10⁻³ M solution of K⁺NCS⁻ in acetonitrile with 0.5 mL of ethyl α-cyanoacrylate causesimmediate (<1 s) solidification accompanied by considerable evolution ofheat. This dramatic response establishes NCS⁻ as an effective initiatorfor the polymerization of ethyl α-cyanoacrylate.

The electronic absorption spectrum of K⁺ R⁻ dissolved in ethylα-cyanoacrylate exhibits bands at 400 nm (ε=89 M⁻¹ cm⁻¹) and 528 nm(ε=114 M⁻¹ cm⁻¹) arising from spin-allowed ligand field transitions.Selective excitation (532 or 546 nm) of the lower-energy transitioncauses the solution to thicken and eventually solidify. This behavior isthe direct consequence of the photochemical release of NCS⁻ which, asnoted above, functions as an effective initiator for polymerization. Thekey role played by anionic species is underscored by the observationthat increased levels of base scavenger (a protonic acid) in the monomersample inhibits polymerization. This effect is attributed to the abilityof the protonic acid to scavenge photogenerated NCS⁻ and/or anionicsites on the growing polymer chain.

EXAMPLE 7

The Effect of Concentration of Anionic Photoinitiator and Incident LightIntensity on Rate of Polymerization

Ethyl α-cyanoacrylate (2 mL) containing various concentrations of K⁺ R⁻were irradiated (with stirring in runs a-c) in a 1-cm rectangular Pyrexcell, as described in Table 7. No attempt was made to exclude air fromthe sample. The dependence of photoinitiated polymerization on the K⁺ R⁻concentration and the incident light intensity is provided in Table 7.At constant intensity, polymerization proceeds more rapidly at highercomplex concentration (compare runs a and c). Such behavior reflects thegreater fraction of light absorbed by the more concentrated sample andthe correspondingly higher photochemical rate of NCS⁻ release. Ananalogous explanation accounts for the faster polymerization observedupon increasing light intensity at constant K⁺ R⁻ concentration (compareruns a and b).

                  TABLE 7    ______________________________________    Effects of K.sup.+ R.sup.-  Concentration and Light Intensity Upon the    Rate of Photoinitiated Polymerization of    Ethyl α-Cyanoacrylate    polymerization    run      [K.sup.+ R.sup.- ], M                       λ.sub.ex, nm                                I, einsteins/s.sup.a                                          time, s.sup.b    ______________________________________    a        2.1 × 10.sup.-3                       546      3.97 × 10.sup.-8                                          880    b        2.1 × 10.sup.-3                       546      2.08 × 10.sup.-8                                          1540    c        5.1 × 10.sup.-4                       546      3.97 × 10.sup.-8                                          3380    d        2.0 × 10.sup.-3                       532      2.13 × 10.sup.-6                                          6    e        6.0 × 10.sup.-4                       532      2.13 × 10.sup.-6                                          12    f        2.0 × 10.sup.-4                       532      2.13 × 10.sup.-6                                          45    ______________________________________     .sup.a Incident light intensity. Light at 546 nm was obtained by passing     the output of a 200W highpressure mercury lamp through a narrow bandpass     interference filter, while a NdYAG pulsed laser operated at 10 Hz was use     for irradiations at 532 nm.     .sup.b Time required for sample to become so viscous that stirring bar     ceased to spin (runs a-c), or time required for the formation of a solid     plug of polymer along the path of the 7mm diameter laser beam (runs d-f).

EXAMPLE 8

Effect of a Free Radical Trap, Hydroquinone, on the PhotoinitiatedAnionic polymerization of Ethyl Cyanoacrylate using Reineckate's Anion

KCr(NH₃)₂ (NCS)₄ (0.0092 grams) was weighed out and dissolved in 10 mlsof ethyl α-cyanoacrylate yielding a solution that was 2.573×10⁻³ M (920ppm) KCr(NH₃)₂ (NCS)₄. This solution was allowed to stir for 60 minutes,in the dark, prior to any photolysis experiments. Subsequently, 3 mls ofthe solution was transferred to a Pyrex test tube equipped with a stirbar. The sample was then photolyzed with the 546 nm filtered output froma 200 W high pressure mercury lamp. The sample polymerized after 446seconds of irradiation.

An additional 3 ml sample of the stock solution was transferred to aPyrex test tube similarly equipped with a stir bar. However, to thissample was added 7 mg of the well-known free radical trap, hydroquinone.That now made this sample 2.573×10⁻³ M or 920 ppm in KCr(NH₃)₂ (NCS)₄and 2.12×10⁻² M or 2,333 ppm in hydroquinone. The 2,333 ppm hydroquinoneconcentration does not take into account the 1000 ppm of hydroquinonepresent in ethyl α-cyanoacrylate as purchased. This sample was thenallowed to stir for 40 minutes prior to photolysis. This sample was thenphotolyzed with the identical set up as the previous sample. The ethylα-cyanoacrylate polymerized after 388 seconds of irradiation. Thehydroquinone clearly had no detrimental effects on the polymerization.

EXAMPLE 9

Effect of a Free Radical Scavenger, Oxygen, on the PhotoinitiatedAnionic Polymerization of Ethyl α-Cyanoacrylate using Reineckate's Anion

KCr(NH₃)₂ (NCS)₄ (0.0082 g) was weighed out and dissolved in 10 mls ofethyl α-cyanoacrylate yielding a solution that was 2.294×10⁻³ M or 820ppm in KCr(NH₃)₂ (NCS)₄. This solution was allowed to stir for 60minutes prior to any photolysis experiments.

After stirring for 60 minutes, 3 mls of the stock solution wastransferred to a Pyrex test tube equipped with a stir bar. This firstsample was not bubbled with any gas and no effort was made to excludeoxygen from the system. The sample was then photolyzed with the 546 nmfiltered output from a 200 W high pressure mercury lamp. After 358seconds of irradiation, the ethyl α-cyanoacrylate had polymerized.

A second 3 ml sample of the initial solution was transferred to a newPyrex test tube and a stir bar was added. The test tube was then cappedwith a rubber septum. Two needles were then forced through the rubberseptum. One long needle was for bubbling the solution with oxygen andone short needle was for venting the excess gas. The solution wasbubbled for ten minutes and then the short needle was removed first andthen the long needle, leaving a static atmosphere of oxygen in the testtube. This sample was then photolyzed using the identical set up justdescribed. The ethyl α-cyanoacrylate polymerized after 398 seconds ofirradiation.

A third sample was prepared with 3 mls of the initial solution and astir bar. The rubber septum was again placed on the test tube and thetwo needles were inserted. However, in this case, the bubbling gas was99.999% pure Argon. The sample was bubbled for 10 minutes and then theneedles were removed. This sample was then photolyzed, with theidentical set up previously described. The ethyl α-cyanoacrylatepolymerized after 321 seconds of irradiation.

Within our experimental error, the percent of free radical scavengingoxygen present had no effect on the rate of polymerizing ethylα-cyanoacrylate. The results of the two sets of experiments justdescribed are clearly not consistent with a free radical process beingresponsible for the polymerization.

EXAMPLE 10

Kinetic Characterization of Anionic Photoinitiation

A more detailed kinetic characterization of the photoinitiatedpolymerization process of ethyl α-cyanoacrylate using Reineckate's anionwas obtained by real-time Fourier-transform infrared spectroscopy. Thisrelatively new technique allows continuous monitoring (up to 204infrared spectra collected per minute) of the sample during irradiation.The extent of polymerization is directly related to the decrease inabsorbance of the C=C stretching band of ethyl α-cyanoacrylate at 1616cm⁻¹. This relationship is expressed in eq. 6, where A₀ denotes theinitial (dark) area of the band and A_(t) is the area after t ##EQU1##seconds of irradiation.

FIG. 3 is a plot of percentage polymerization vs. time of irradiationfor samples of ethyl α-cyanoacrylate containing the followingconcentrations of Reineckate's anion: (□) O M; (*) 2.2×10⁻⁴ M; (+)5.4×10⁻⁴ M; (▪) 2.0×10⁻³ M. Samples were coated onto acid-treatedsilicon wafers and irradiated with 500 mW/cm² of polychromatic (300-630nm) light.

The data shown in FIG. 1 agree with earlier observations (Table 7) thatpolymerization occurs more rapidly in samples containing higher K⁺ R⁻concentrations. The rate of photoinitiated polymerization, R_(p), can becalculated from eq. 7, where A_(t1) and A_(t2) represent the areas underthe 1616 cm⁻¹ band at irradiation times t1 and t2, and M is the molarconcentration of vinyl groups in ethyl α-cyanoacrylate. Maximum rates of3.0, 1.3, and 0.60 M·s⁻¹ result for ethyl α-cyanoacrylate samplescontaining 2.0×10⁻³, 5.4×10⁻⁴, and 2.2×10⁻⁴ M concentrations of K⁺ R⁻,respectively. ##EQU2##

This data demonstrates that visible-light irradiation of trans-[Cr(NH₃)₂(NCS)₄ ]⁻ dissolved in ethyl α-cyanoacrylate initiates anionicpolymerization. The initiation step involves addition of photoreleasedNCS⁻ to the carbon-carbon double bond of the acrylate monomer.

This invention has been described with reference to its preferredembodiments. Variations and modifications of the invention, method foranionic polymerization, will be obvious to those skilled in the art fromthe foregoing detailed description of the invention. It is intended thatall of these variations and modifications be included within the scopeof the appended claims.

We claim:
 1. A method for polymerizing a monomer, or crosslinking orpolymerizing an oligomer or crosslinking a polymer by anionicphotoinitiation comprising irradiating a mixture of the monomer,oligomer, or polymer, which is selected from the group consisting ofethylene, 1,3-diene, styrene, α-methylstyrene, acrylate ester,methacrylate ester, acrylonitrile, methacrylonitrile, acrylamide,methacrylamide, aldehydes, ketones, N-carboxy-α-amino anhydrides, cyclicamides, cyclic esters, epoxides, and siloxanes, and an inorganictransition metal complex Pt(acac)₂, under irradiation conditions whereinthe inorganic complex photochemically reacts to release an anionicallycharged nucleophile and said anionically charged nucleophile initiates apolymerization reaction of said monomer, or a crosslinking orpolymerization reaction of said oligomer, or a crosslinking reaction ofsaid polymer.
 2. The method of claim 1, wherein the monomer, oligomer,or polymer is selected from the group consisting of ethylene, 1,3-diene,styrene, α-methyl styrene, acrylate ester, methacrylate ester,acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, aldehydes,ketones and oligomers or polymers of said monomers.
 3. The method ofclaim 1, wherein the monomer, oligomer, or polymer is selected from thegroup consisting of N-carboxy-α-amino anhydrides, cyclic amides, cyclicesters, epoxides, siloxanes and oligomers or polymers of said monomers.4. The method of claim 1, wherein said acrylate ester monomer is2-cyanoethylacrylate.
 5. The method of claim 1, wherein a polymer oroligomer is crosslinked by said photoinitiation.
 6. The method of claim1, wherein an oligomer or monomer is polymerized by saidphotoinitiation.
 7. The method of claim 1, wherein said polymerizationor crosslinking occurs at a temperature between 20° and 30° C.
 8. Themethod of claim 1, wherein the radiation is near-ultraviolet or visiblelight.
 9. The method of claim 1, wherein the inorganic complex isthermally stable.
 10. The method of claim 1, wherein the irradiationcauses a ligand field or ligand-to-metal charge transfer transition inthe inorganic complex.
 11. The method of claim 1, further comprisingforming the crosslinked or polymerized monomer, oligomer, or polymerinto a material selected from the group consisting of lithographicphotoresists, stereolithographic media, photocurable coatings,adhesives, sealants and printing inks.